Ab initio CCSD(T) and MRD-CI study of excited states and the electronic spectrum of linear C-5(+)

Large-scale ab initio coupled cluster and multi-reference configuration int eraction calculations (MRD-CI) are carried out to determine the equilibrium geometry and the vertical electronic spectrum of linear C-5(+). Contrary t o prior theoretical estimates we find three low-lying states within an ener gy range of 0.3 eV: (2)Sigma (+)(u), (2)Sigma (+)(g) and (2)Pi (g) and a sy mmetric arrangement of nuclei. Transitions from (2)Sigma (+)(u) to these lo w-lying states are dipole-allowed; sizeable oscillator strengths are comput ed for the 2 Pi (+)(g) <-- X(2)Sigma (+)(u): transition at 2.62 eV and the (g)Sigma (g) <-- X(2)Sigma (+)(u) transition at 3.36 eV and should give a g uide to spectroscopic identification of linear C-5(+). (C) 2001 Elsevier Sc ience B.V. All rights reserved.